1. Field of the Invention
The present invention relates to an impedance conversion device that is to be provided in an antenna device, an antenna device provided with the impedance conversion device, and a communication terminal device that includes the antenna device. More specifically, the present invention relates to a technology that provides an antenna device that performs matching over a wide frequency band.
2. Description of the Related Art
In recent years, there has been a demand for communication terminal devices, such as cellular phones, to be compatible with communication systems, such as global system for mobile communication (GSM) (registered trademark), digital communication system (DCS), personal communication services (PCS), and universal mobile telecommunications system (UMTS), and in addition, to be compatible with, for example, global positioning system (GPS), wireless LANs and Bluetooth (registered trademark). Therefore, there has been a demand for the antenna device of such a communication terminal device to cover a wide frequency band from around 700 MHz to around 2.7 GHz.
Antenna devices that cover a wide frequency band are generally equipped with a wide-band matching circuit including an LC parallel resonant circuit and an LC series resonant circuit as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697. In addition, tunable antennas, such as those disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065, are also known examples of antenna devices that cover a wide frequency band.
However, since the matching circuits disclosed in Japanese Unexamined Patent Application Publication No. 2004-336250 and Japanese Unexamined Patent Application Publication No. 2006-173697 include a plurality of resonant circuits, the insertion loss of the matching circuit is likely to be large and a sufficient gain will not obtained.
On the other hand, the tunable antennas disclosed in Japanese Unexamined Patent Application Publication No. 2000-124728 and Japanese Unexamined Patent Application Publication No. 2008-035065 require a circuit for controlling a variable capacitance element, that is, a switching circuit for switching between frequency bands and, therefore, the circuit configuration is complicated. In addition, since the loss and strain are large in a switching circuit, it is possible that sufficient gain will not be obtained.